The present invention relates to multi-stage axial flow turbines and particularly relates to apparatus and methods for minimizing the vibrational stress in downstream turbine blades caused by asymmetrical flow in the turbine.
In a conventional multi-stage axial flow turbine, for example, a steam turbine, gas flows through the rotor blades and stator blades of each stage in a generally axial direction. Turbine blade lengths typically increase in radial length in the downstream direction of flow such that the turbine blades in the later stages have substantial radial lengths. In a conventional steam turbine, asymmetrical flow is often input through a first-stage nozzle plate. That is, the flow is not input to the turbine 360.degree. about its axis. Rather, the input is asymmetrical, for example, on the order of 180.degree. about the turbine axis or at multiple flow inputs asymmetrically arranged about the turbine axis. In the instance of an asymmetrical input of about 180.degree., it will be appreciated that the flow profile exiting the asymmetrically arranged first-stage nozzles looking axially upstream toward the nozzles would resemble a square wave, with the width of the wave proportional to the nozzle arc length. As the flow passes through the second stage, the asymmetric flow pattern about the turbine axis attenuates somewhat but remains asymmetrical. That is, it tends towards symmetry about the turbine axis. As the flow passes through succeeding stages, the attenuation of the asymmetrical flow continues until the flow reaches the last stage. Depending upon the strength and shape of the flow pattern exiting the first stage, as well as the number of stages between the first stage and the last stage, the resulting flow pattern entering the last stage or stages can result in excessive vibration stresses on the radially longer turbine blades of those downstream stages due to residual asymmetrical flow patterns acting on those long blades.
The interaction between the natural frequencies of the turbine blades and the nozzle excitation is well known. It has been common practice to consider such interaction at various running speeds to ensure high turbine reliability. That is, when the turbine runs at different speeds, it is possible to run the turbine at a speed that will excite the natural resonance of the turbine blades leading to excessive vibrational stresses. It will be appreciated that the turbine blades vibrate at different frequencies depending upon their shape and the speed of the turbine. It has been discovered, however, that the asymmetry of the flow, i.e., partial arc flow disturbances, present in the first stage of a turbine can filter through the entire axial flow path and influence the vibration stresses in the longer turbine blades of the last stage or stages. Actual practice has demonstrated that at certain running speeds, the asymmetrical flow pattern introduced in the early stages of the turbine can cause the long turbine blades in the later turbine stages to vibrate at their natural frequency, causing stress on the blades. If the vibration has a sufficient magnitude, the blades may break.
According to the present invention, apparatus and methods are provided for positively attenuating the asymmetric axial flow through the turbine such that the flow tends toward axi-symmetric flow in the later stages of the turbine. Thus, in accordance with the present invention, a flow baffle is provided downstream of the first stage nozzle plate through which the asymmetrical flow is introduced into the turbine. Particularly, the baffle is disposed before the stator blades of the next stage downstream from the introduction of the asymmetrical flow through the first-stage nozzle plate. The baffle extends annularly about the stator blading, causing the asymmetric flow to flow about the baffle before it is introduced into the second-stage nozzles. This tends to direct the flow about the turbine axis. Consequently, the asymmetric flow pattern is positively attenuated toward an axi-symmetric flow pattern by the baffle. By positively attenuating the initial asymmetrical flow at an early stage in the turbine, the flow pattern in the later stages obtains greater symmetry about the turbine axis than otherwise, thereby reducing or minimizing the vibrational stress imposed on the later-stage longer turbine blades caused by turbine blade response to asymmetrical flow. Stated differently, because the natural frequency of vibration of the radially longer blades of the later turbine stages can be excited by asymmetrical flow, the input asymmetrical flow pattern is positively directed to a substantially symmetrical flow in the later stages or at least an asymmetrical flow pattern which does not cause harmful vibrational stresses and does not cause the turbine blading to resonate at its natural frequency.
It will also be appreciated that, in accordance with the present invention, the flow attenuation is not limited to asymmetrical flow disturbances occurring between the first and second stages of the turbine. In many turbines, asymmetrical flow is introduced into intermediate stages of the turbine. Where this is done, a baffle according to the present invention may be provided at the next stage downstream from the intermediate asymmetric flow disturbance to attenuate the asymmetry of that flow toward axi-symmetric flow in the later stages of the turbine.
In a preferred embodiment according to the present invention, there is provided a multi-stage axial flow turbine, comprising a plurality of turbine stages arranged in the direction of axial flow with each stage including a plurality of stator blades and a plurality of rotor blades, means carried by the turbine for introducing gas under pressure asymmetrically about the axis of the turbine into the turbine stages and a baffle disposed downstream of the introducing means for attenuating the asymmetry of the gas flow in the turbine stages downstream of the baffle.
In a further preferred embodiment according to the present invention, there is provided a multi-stage axial flow turbine, comprising a plurality of turbine stages arranged in the direction of axial flow with each stage including a plurality of stator blades and a plurality of rotor blades, the rotor blades of subsequent stages having blades radially longer in length than the length of the rotor blades in preceding stages, means carried by the turbine for introducing asymmetrical flow into the turbine stages and a baffle disposed downstream of the introducing means for attenuating the asymmetry of the flow in turbine stages downstream of the baffle to prevent vibration of the longer rotor blades substantially at their natural resonant frequencies as a result of the asymmetrical flow. In a further aspect of this invention, there is provided, in a multi-stage axial flow turbine having a plurality of turbine stages arranged in the direction of axial flow, a method of minimizing vibration stresses in rotor blades downstream of an asymmetrical flow input to the turbine comprising the step of attenuating the asymmetrical flow by directing the flow toward an axi-symmetric flow downstream of the asymmetrical flow input.
Accordingly, it is a primary object of the present invention to provide apparatus and methods for minimizing vibrational stresses in downstream turbine blades in response to asymmetrical flow input to the turbine.
These and further objects and advantages of the present invention will become more apparent upon reference to the following specification, appended claims and drawings.